You go up to the magnifying glass icon on the instance that you want to subscribe on (not the instance that the magazine or community actually belongs to), then that's where you'd search washingtondc@lemmit.online

It should appear and you should be able to follow it. If you are the first person to look up that magazine on your instance it may say that there are zero posts and that the owner of the magazine is your instance's admin, but as long as the name and the magazine/community icon looks like you're expecting you can follow it and it'll start federating in new posts over time.